An unconventional interaction between Dis1/TOG and Mal3/EB1 promotes the fidelity of chromosome segregation

Abstract

Dynamic microtubule plus ends interact with various intracellular target regions such as the cell cortex and the kinetochore. Two conserved families of microtubule plus-end tracking proteins, XMAP215/TOG and EB1, play pivotal roles in regulating microtubule dynamics. Here we study the functional interplay between fission yeast Dis1/XMAP215 and Mal3/EB1. Using an in vitro microscopy assay, we find that purified Dis1 autonomously tracks growing microtubule ends and is a bona fide microtubule polymerase. Mal3 recruits additional Dis1 to microtubule ends, explaining the synergistic enhancement of microtubule dynamicity by these proteins. A non-canonical binding motif in Dis1 mediates the interaction with Mal3. X-ray crystallography shows that this novel motif interacts in an unconventional configuration with the conserved hydrophobic cavity formed within the Mal3 C-terminal region that typically interacts with the canonical SXIP motif. Selectively perturbing the Mal3-Dis1 interaction in living cells demonstrates that it is important for accurate chromosome segregation. Whereas in some metazoans the EB1-XMAP215/TOG interaction requires an additional binding partner, fission yeast relies on a direct interaction, indicating evolutionary plasticity of this critical interaction module.